Conventional internal combustion engines operating on either a two-stroke or a four-stroke cycle commonly use a crankshaft and con rod arrangement to convert linear motion of a piston to rotary motion at an output shaft. Due to the crankshaft and con rod geometry, maximum piston acceleration generally occurs when the piston is at top dead centre (TDC), where piston acceleration is significantly greater than at bottom dead centre (BDC). This reduction in TDC dwell time (time spent at or near TDC) has several negative effects, including reduced efficiency and engine unbalancing.
Several alternative engine arrangements are known which use different combustion chamber to output shaft coupling mechanisms to reduce maximum piston acceleration and increase TDC dwell time. However, these are generally complex and are difficult and expensive to manufacture.
In addition to the problems mentioned above, conventional two-stroke engines also suffer from problems with lubrication of the crankshaft and con rod assembly. The crankshaft and con rod assembly is generally housed within a crank case forming part of the induction system. The lubrication system operates as a total loss system in which lubricating oil is continuously fed into the crank case and allowed to pass into the combustion cylinder and thence out of the engine. This total loss lubricating system is damaging to the environment due to the presence of lubricating oil in the exhaust gases. The use of the crank case as a supercharging or induction chamber also limits the ability of engine designers to optimise the volume and shape of the induction chamber to maximise performance and efficiency of the engine.
Conventional crankshaft and con-rod two and four stroke internal combustion engines in general favour having a long piston travel in relation to its diameter to provide greater thermodynamic efficiency and to meet ever increasingly stringent emissions criteria. The longer piston travel in turn requires a greater crank eccentricity, which when combined with the space occupied by the moving conrod produces a correspondingly large frontal area in the axis of the crankshaft.
This frontal area in the axis of the crankshaft is often increased by configurations or mechanisms to provide vibrational balance—for example offsetting multiple cylinders in a V shape or opposed piston shape or the addition of counterbalancing flywheels or shafts.
This orthodox design approach, starting with the optimization of the engine, makes it fundamentally difficult or unattractive to submerge the engine in water (in the case of an outboard motor) because of the increased drag or drag in air in the case of aircraft or bulkiness in the case of motorbikes and other forms of transport, and other applications such as generators, range extenders, garden tools, etc., where use of space is an important design consideration and requires optimization.
Outboard motors for water craft, particularly the portable end of the market requiring power outputs generally below around 20 horse power, currently employ relatively inexpensive four-stroke engines. Tighter emissions regulations have reduced the use of two stroke engines at lower powers (in fact it is now illegal to sell many two stroke outboard engines in certain countries) and favoured the development of cleaner four strokes.